I have a recent 625JM. I've fired 1500-2000 rounds thru it, 95%+ were my cast lead bullets sized 0.452. The bore measures 0.451, the smallest cylinder measure 0.452. My hardness is low at 10.4+/-bhn. Use the largest cast bullet that fits the smallest cylinder and your leading will go away. Ideal size is .001-.002 over the barrel size, but no larger than the smallest hole in the cylinder.